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1. An incubation method to determine the age of available nonstructural carbon in woody plant tissues

   https://doi.org/10.1093/treephys/tpad015  

   Peltier, Drew M. P.; Lemoine, Jim; Ebert, Chris; Xu, Xiaomei; Ogle, Kiona; Richardson, Andrew D.; Carbone, Mariah S.; Landhäusser, ed., Simon (February 2023, Tree Physiology)

   Abstract Radiocarbon (∆14C) measurements of nonstructural carbon enable inference on the age and turnover time of stored photosynthate (e.g., sugars, starch), of which the largest pool in trees resides in the main bole. Because of potential issues with extraction-based methods, we introduce an incubation method to capture the ∆14C of nonstructural carbon via respired CO2. In this study, we compared the ∆14C obtained from these incubations with ∆14C from a well-established extraction method, using increment cores from a mature trembling aspen (Populus tremuloides Michx). To understand any potential ∆14C disagreement, the yields from both methods were also benchmarked against the phenol-sulfuric acid concentration assay. We found incubations captured less than 100% of measured sugar and starch carbon, with recovery ranging from ~ 3% in heartwood to 85% in shallow sapwood. However, extractions universally over-yielded (mean 273 ± 101% expected sugar carbon; as high as 480%), where sugars represented less than half of extracted soluble carbon, indicating very poor specificity. Although the separation of soluble and insoluble nonstructural carbon is ostensibly a strength of extraction-based methods, there was also evidence of poor separation of these two fractions in extractions. The ∆14C of respired CO2 and ∆14C from extractions were similar in the sapwood, whereas extractions resulted in comparatively higher ∆14C (older carbon) in heartwood and bark. Because yield and ∆14C discrepancies were largest in old tissues, incubations may better capture the ∆14C of nonstructural carbon that is actually metabolically available. That is, we suggest extractions include metabolically irrelevant carbon from dead tissues or cells, as well as carbon that is neither sugar nor starch. In contrast, nonstructural carbon captured by extractions must be respired to be measured. We thus suggest incubations of live tissues are a potentially viable, inexpensive and versatile method to study the ∆14C of metabolically relevant (available) nonstructural carbon. 

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2. Climate control on terrestrial biospheric carbon turnover

   https://doi.org/10.1073/pnas.2011585118  

   Eglinton, Timothy I.; Galy, Valier V.; Hemingway, Jordon D.; Feng, Xiaojuan; Bao, Hongyan; Blattmann, Thomas M.; Dickens, Angela F.; Gies, Hannah; Giosan, Liviu; Haghipour, Negar; et al (February 2021, Proceedings of the National Academy of Sciences)

   Terrestrial vegetation and soils hold three times more carbon than the atmosphere. Much debate concerns how anthropogenic activity will perturb these surface reservoirs, potentially exacerbating ongoing changes to the climate system. Uncertainties specifically persist in extrapolating point-source observations to ecosystem-scale budgets and fluxes, which require consideration of vertical and lateral processes on multiple temporal and spatial scales. To explore controls on organic carbon (OC) turnover at the river basin scale, we present radiocarbon ( 14 C) ages on two groups of molecular tracers of plant-derived carbon—leaf-wax lipids and lignin phenols—from a globally distributed suite of rivers. We find significant negative relationships between the 14 C age of these biomarkers and mean annual temperature and precipitation. Moreover, riverine biospheric-carbon ages scale proportionally with basin-wide soil carbon turnover times and soil 14 C ages, implicating OC cycling within soils as a primary control on exported biomarker ages and revealing a broad distribution of soil OC reactivities. The ubiquitous occurrence of a long-lived soil OC pool suggests soil OC is globally vulnerable to perturbations by future temperature and precipitation increase. Scaling of riverine biospheric-carbon ages with soil OC turnover shows the former can constrain the sensitivity of carbon dynamics to environmental controls on broad spatial scales. Extracting this information from fluvially dominated sedimentary sequences may inform past variations in soil OC turnover in response to anthropogenic and/or climate perturbations. In turn, monitoring riverine OC composition may help detect future climate-change–induced perturbations of soil OC turnover and stocks. 

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3. The Impacts of Flood, Drought, and Turbidites on Organic Carbon Burial Over the Past 2,000 years in the Santa Barbara Basin, California

   https://doi.org/10.1029/2020PA003849  

   Sarno, Caitlyn T.; Benitez‐Nelson, Claudia R.; Ziolkowski, Lori A.; Hendy, Ingrid L.; Davis, Catherine V.; Tappa, Eric J.; Thunell, Robert C. (July 2020, Paleoceanography and Paleoclimatology)

   Abstract Climate conditions and instantaneous depositional events can influence the relative contribution of sediments from terrestrial and marine environments and ultimately the quantity and composition of carbon buried in the sediment record. Here, we analyze the elemental, isotopic, and organic geochemical composition of marine sediments to identify terrestrial and marine sources in sediment horizons associated with droughts, turbidites, and floods in the Santa Barbara Basin (SBB), California, during the last 2,000 years. Stable isotopes (δ¹³C and δ¹⁵N) indicate that more terrestrial organic carbon (OC) was deposited during floods relative to background sediment, while bulk C to nitrogen (C/N) ratios remained relatively constant (~10). Long‐chainn‐alkanes (C₂₇, C₂₉, C₃₁, and C₃₃), characteristic of terrestrial OC, dominated all types of sediment deposition but were 4 times more abundant in flood layers. Marine algae (C₁₅, C₁₇, and C₁₉) and macrophytes (C₂₁and C₂₃) were also 2 times higher in flood versus background sediments. Turbidites contained twice the terrestrialn‐alkanes relative to background sediment. Conversely, drought intervals were only distinguishable from background sediment by their higher proportion of marine algaln‐alkanes. Combined, our data indicate that 15% of the total OC buried in SBB over the past 2,000 years was deposited during 11 flood events where the sediment was mostly terrestrially derived, and another 12% of deep sediment OC burial was derived from shelf remobilization during six turbidite events. Relative to twentieth century river runoff, our data suggest that floods result in considerable terrestrial OC burial on the continental margins of California. 

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4. Exploring the Increasing Trend of Organic Carbon in the Atmospheric Aqueous Phase

   Lawrence, C; Lance, S; Casson, P; Tripathy, A; Brandt, R; Synder, P; Kelting, D; Yerger, E; Murray, G; Campbell, J; et al (November 2024, National Atmospheric Deposition Program)

   Organic carbon (OC) is a highly diverse class of compounds that represents a small but critical fraction of the atmosphere’s chemical composition. Volatile organic compounds (VOCs), when combined with nitrogen oxides (NOx), can produce tropospheric ozone (O3), a regulated air pollutant. OC also represents a large and growing fraction of aerosol mass, either through direct emissions from sources like fossil combustion and biomass burning, or through secondary chemistry by the oxidation and subsequent reduction of vapor pressure of VOCs leading to condensational growth. Clouds droplets and precipitation can contain additional OC due to the dissolution of soluble organic gases to the aqueous phase. OC has abundantly been found in aqueous samples of clouds, fog, and precipitation, exposing these compounds to unique aqueous chemical reactions and wet deposition. However, the concentrations and controlling factors of atmospheric aqueous organic carbon remain highly unconstrained. Cloud water measurements at Whiteface Mountain in the Adirondack Mountains in upstate New York have revealed an increasing trend of Total Organic Carbon (TOC), with annual median concentrations doubling in 14 years, possibly signaling a growing trend in atmospheric OC. However, the causes and potential consequences of this trend remain unclear. Another question that has yet to be explored is if this trend in OC extends beyond WFM. To answer this question, this work explores the trends of WFM cloud water and 4 additional long-term cloud water and wet deposition datasets that have measured TOC or dissolved OC (DOC) throughout the Northeast US. These sites include Mt Washington, NH, Hubbard Brook NH, Thompson Farm NH, and Sleepers River Vermont. This work will also discuss potential hypotheses driving this increasing trend including increased biomass burning influence and increased biogenic emissions in the region. 

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5. Thermal oxidation of carbon in organic matter rich volcanic soils: insights into SOC age differentiation and mineral stabilization

   https://doi.org/10.1007/s10533-019-00586-1  

   Grant, Katherine E.; Galy, Valier V.; Chadwick, Oliver A.; Derry, Louis A. (August 2019, Biogeochemistry)

   Radiocarbon ages and thermal stability measurements can be used to estimate the stability of soil organic carbon (OC). Soil OC is a complex reservoir that contains a range of compounds with different sources, reactivities, and residence times. This heterogeneity can shift bulk radiocarbon values and impact assessment of OC stability and turnover in soils. Four soil horizons (Oa, Bhs, Bs, Bg) were sampled from highly weathered 350 ka Pololu basaltic volcanics on the Island of Hawaii and analyzed by Ramped PyrOX (RPO) in both the pyrolysis (PY) and oxidation (OX) modes to separate a complex mixture of OC into thermally defined fractions. Fractions were characterized for carbon stable isotope and radiocarbon composition. PY and OX modes yielded similar results. Bulk radiocarbon measurements were modern in the Oa horizon (Fm = 1.013) and got progressively older with depth: the Bg horizon had an Fm value of 0.73. Activation energy distributions (p(E)) calculated using the ‘rampedpyrox’ model yielded consistent mean E values of 140 kJ mol-1 below the Oa horizon. The ‘rampedpyrox’ model outputs showed a mostly bimodal distribution in the p(E) below the Oa, with a primary peak at 135 kJ mol-1 and a secondary peak at 148 kJ mol-1, while the Oa was dominated by a single, higher E peak at 157 kJ mol-1. We suggest that mineral-carbon interaction, either through mineral surface-OC or metal-OC interactions, is the stabilization mechanism contributing to the observed mean E of 140 kJ mol-1 below the Oa horizon. In the Oa horizon, within individual RPO analyses, radiocarbon ages in the individual thermal fractions were indistinguishable (p[0.1). The flat age distributions indicate there is no relationship between age and thermal stability (E) in the upper horizon ([25 cm). Deeper in the soil profile higher lEf values were associated with older radiocarbon ages, with slopes progressively steepening with depth. In the deepest (Bg) horizon, there was the largest, yet modest change in Fm of 0.06 (626 radiocarbon years), indicating that older OC is slightly more thermally stable. 

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